Surrounding the swollen roots of sweetpotato with a decomposing band of an organic amendment enhances nematode-suppressive services and reduces damage caused by root-knot nematode

A previous field trial showed that marketable yield increased and root-knot nematode populations declined when the swollen roots of sweetpotato were surrounded by a sawdust-based amendment. Further evidence that such amendments provide worthwhile levels of nematode control was obtained in this study. Customised pots filled with an organic amendment or amended soil were kept in a greenhouse or buried in the field for various times and then the nematode-control potential of various treatments was assessed by surrounding the pots with soil infested with Meloidogyne incognita . Swollen root production generally increased, and root-knot nematode populations consistently declined, when sawdust was used as an amendment. The addition of an organic nitrogen source generally improved the level of control. Laboratory and greenhouse assays confirmed that amended soils were biologically suppressive, as root-knot nematode numbers were lower and fewer galls were produced in soil containing decomposing amendments than in soil or sand that had been heated to kill the soil biota. A predatory nematode morphologically similar to Clarkus papillatus was one of the suppressive agents as numbers were very high in some experiments. Mesostigmatid mites in the family Laelapidae and genera Sejus , Protogamasellus , Holaspulus and Gamasiphis were also contributing, while an acarid mite ( Rhizoglyphus robini ) occurred in high numbers and was observed devouring root-knot nematode juveniles on agar plates. Nematophagous fungi were also involved as three species of nematode-trapping fungi ( Arthrobotrys musiformis , A. thaumasia and A. oligospora ) and an unidentified fungus that captures nematodes on bulbous hyphal extensions were consistently isolated from amended soils. These results suggest that sawdust-based amendments enhance the nematode-suppressive services provided by a wide range of predators.